One method of classifying polyethylenes is by density. A high-density polyethylene (HDPE) has a density ranging from about 0.941 g/cm3 to about 0.970 g/cm3, a medium-density polyethylene (MDPE) from about 0.926 g/cm3 to about 0.940 g/cm3; and a low-density polyethylene (LDPE) from about 0.910 g/cm3 to about 0.925 g/cm3. Subcategories of LDPE include linear low density polyethylene (LLDPE) and very low density polyethylene (VLDPE). LLDPE has a density ranging from about 0.916 g/cm3 to about 0.930 g/cm3 and VLDPE has a density ranging from about 0.890 g/cm3 to about 0.915 g/cm3.
LLDPE and VLDPE are prepared by copolymerizing ethylene and 1-olefins. The incorporation of 1-olefins results in short chain branching (SCB) of 10 carbon atoms or less. A range of properties in polyethylenes can be obtained by controlling the amount of SCB and the short-chain branching distribution (SCBD). SCBD is a measure of the distribution of short chain branches across the width of the copolymer molecular weight distribution (MWD). A more uniform or homogenous distribution of SCB can lead to improved mechanical properties.
Multi-site catalysts, e.g. Ziegler, and single-site catalysts, e.g. metallocene, are commonly used to prepare LLDPE and VLDPE. While single-site catalysts, when compared to Ziegler catalysts, are known to produce polyethylenes with greater monomer incorporation, more uniform SCBD and a narrower MWD, the polyethylenes have been known to suffer from poor processability. Polyethylenes prepared with Ziegler catalysts exhibit good processability. However, the lower amount and less uniform SCB can adversely affect physical properties, such as dart-drop strength in film applications.
Methods of preparing polyethylenes that take advantage of the benefits of both single- and multi-site catalysts are known. One method is to blend two or more polyethylenes having different properties before or during processing. For example, a polyethylene prepared with a single-site catalyst and a polyethylene prepared with a Ziegler catalyst can be melt blended to produce a polyethylene blend. One difficulty with melt blending is that it can be difficult to produce polyethylene blends that are homogenous.
Another approach is to use two catalysts in one or more reactors to form a bi- or multimodal polyolefin. For example two single-site catalysts or a single-site and a Ziegler catalyst can be used, see e.g., U.S. Pat. Nos. 4,937,299, 4,530,914, 5,032,562 and 5,539,076. Processes using two catalysts are more complicated and single-site catalysts can be more expensive than Ziegler catalysts.
While some processes employ two Ziegler catalysts in a multi-reactor polymerization, the resulting polyethylene can suffer from the same problems associated with Ziegler catalysts, e.g. low levels of SCB, as discussed above.
In sum, new processes are needed. Particularly valuable processes would use one catalyst to produce a polyethylene that has the attributes of a polyethylene prepared with a single-site and Ziegler catalyst.